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The Sensitivity of Gas Hydrate Reservoirs to Climate Change

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The Sensitivity of Gas Hydrate Reservoirs to Climate Change The sensitivity of gas hydrate reservoirs to climate change: Perspectives from a new combined model for permafrost-related and marine settings Thomas Mestdagh, Jeffrey Poort, Marc de Batist To cite this version: Thomas Mestdagh, Jeffrey Poort, Marc de Batist. The sensitivity of gas hydrate reservoirs to climate change: Perspectives from a new combined model for permafrost-related and marine settings. Earth- Science Reviews, Elsevier, 2017, 169, pp.104-131. 10.1016/j.earscirev.2017.04.013. hal-01521071 HAL Id: hal-01521071 https://hal.sorbonne-universite.fr/hal-01521071 Submitted on 11 May 2017 HAL is a multi-disciplinary open access L’archive ouverte pluridisciplinaire HAL, est archive for the deposit and dissemination of sci- destinée au dépôt et à la diffusion de documents entific research documents, whether they are pub- scientifiques de niveau recherche, publiés ou non, lished or not. The documents may come from émanant des établissements d’enseignement et de teaching and research institutions in France or recherche français ou étrangers, des laboratoires abroad, or from public or private research centers. publics ou privés. ACCEPTED MANUSCRIPT The sensitivity of gas hydrate reservoirs to climate change: perspectives from a new combined model for permafrost-related and marine settings Thomas Mestdagh a,*, Jeffrey Poort b, Marc De Batist a a Renard Centre of Marine Geology, Department of Geology, Ghent University, Krijgslaan 281 (S8), 9000 Ghent, Belgium b Institut des Sciences de la Terre de Paris, Université Pierre et Marie Curie, Case courrier 129-4, place Jussieu, 75252 Paris, France * Corresponding author: [email protected] Abstract Gas hydrate reservoirs store large quantities of gas in sediments on continental margins, in deep lakes, and in continental and relic sub-shelf permafrost. The gas hydrate structure is only stable at sufficiently low temperature and high pressure, and may therefore collapse under changing climatic conditions. If a temperature rise or pressure drop (e.g. through falling sea level) is effective enough to dissociate hydrate deposits, methane (the most common gas component in hydrates and a potent greenhouse gas) is released from the hydrate structure and may eventually enter into the atmosphere. This may generate a positive feedback effect, as resulting enhanced greenhouse gas levels would additionally warm the atmosphere and hence maintain or reinforce hydrate dissociation. The significance of this mechanism has been debated over the past decades, often within the framework of geologically rapid Quaternary climatic oscillations and present-day climate warming. An extensive set of studies has addressed the climate-sensitivity of gas hydrate reservoirs in various study areasACCEPTED and geological settings, MANUSCRIPT and by means of various approaches. No real consensus has yet been reached on the matter. In this study, we seek to evaluate the sensitivity of gas hydrate reservoirs to changes in global climate from a more general perspective, by firstly reviewing the available literature, and secondly developing a new numerical model to quantify gas hydrate destabilization under changing environmental conditions. Qualities of the model include the wide applicability to both marine and permafrost-related hydrate reservoirs and the integrative 1 ACCEPTED MANUSCRIPT approach, combining existing hydrate formation models with a dissocation model that accounts for the consumption of latent heat during hydrate dissociation. To determine which settings are most vulnerable, and to acquire insight into the extent, fashion and rates of hydrate dissociation, we apply the model to four distinct types of hydrate reservoirs across a hypothetic high-latitude continental margin under two specific cases of climate change: the last deglaciation following the Last Glacial Maximum and present-day climate warming. The simulations indicate that hydrates on the upper continental slope and in association with thin, sub-shelf permafrost are most sensitive to the imposed climatic variations, whereas deepwater and onshore permafrost-related reservoirs react in a more stable way. However, the deep (i.e. at several tens to hundreds of meters subsurface depth) stratigraphic-type hydrates considered in this study constitute by far the largest fraction of the global gas hydrate volume, but dissociate on slow timescales of thousands to hunderds of thousands of years, even in the most sensitive environments. In contrast, shallow (i.e. at, or a few meters below the surface or seafloor) structural-type hydrates are able to respond to climatic variations on sub- millennial timescales, but the volumes of gas they may release are probably insignificant to the global carbon cycle and climate. Quaternary and present-day climate change do affect the stability of gas hydrate reservoirs, but at long timescales where hydrate volumes are large, and on short timescales where hydrate volumes are small. Consequently, gas hydrates dissociate to an extent that is too small or at a pace that is too slow to create a strong positive feedback effect. While the release of methane from the disintegration of gas hydrates is observed on different margins today, it is not likely to have played a leading role in Quaternary climatic variations or to become a significant process in the comingACCEPTED centuries as a result of present MANUSCRIPT-day rising temperatures. Keywords: gas hydrate; climate change; methane; modeling; marine; permafrost 2 ACCEPTED MANUSCRIPT 1. Introduction Gas hydrates are ice-like crystalline compounds of water and gas (in nature predominantly methane), that occupy the pore space of sediments if low temperature and high pressure conditions prevail, and if a sufficiently large volume of gas is available (Davie and Buffett, 2001). Natural gas hydrates have been discovered in marine sediments on continental margins, in the sedimentary infill of deep lake basins, as well as in association with continental and relic sub-shelf permafrost (Collett et al., 2009). Milkov (2004) estimates the global volume of natural gas contained in submarine hydrate reservoirs to be in the range of 1 to 5 × 1015 m3 (equivalent to ± 500 - 2500 Gton of methane carbon). In addition to this, about 20 Gton of carbon is estimated to be stored in Arctic permafrost-related hydrate deposits (Ruppel, 2015). This vast amount of methane, locked up in hydrate reservoirs at relatively shallow depths in the geosphere (compared to regular natural gas accumulations), has drawn the attention of the research community because it may constitute a potential energy resource (Koh et al., 2012), and because it may play a role in the global carbon cycle and influence the Earth’s climate (e.g. Nisbet, 1990; Dickens et al., 1995; Kvenvolden, 2002; Kennett et al., 2003; Archer, 2007; Archer et al., 2009; Ruppel, 2011; Ruppel and Kessler, 2017). Awareness of the latter issue originated in the 1980’s, with a.o. Kvenvolden (1988) realizing that gas hydrate reservoirs may release significant amounts of methane (CH4) and its oxidation product carbon dioxide (CO2), both greenhouse gases, to the atmosphere, when hydrates decompose as a result of global warming. The subsequent increase in atmospheric greenhouse gas levels would contribute to already warming temperatures and thus maintain or even amplify the dissociation of the hydrates. As such, this positive feedback ACCEPTEDmechanism would mark a MANUSCRIPTclose interrelationship between geologically rapid climate change and fluctuations in atmospheric greenhouse gas levels on the one hand, and variations in the size of the global gas hydrate reservoir on the other (figure 1). Hydrate reservoirs in high-latitude areas are believed to be most sensitive to this process, because climatic changes are most pronounced here (e.g. IPCC, 2013) and hydrates exist below shallower water depths. Hypotheses relying on this hydrate-climate coupling have been formulated within the framework of 3 ACCEPTED MANUSCRIPT Quaternary ‘glacial to interglacial’ transitions (e.g. Nisbet, 1990; Paull et al., 1991; Loehle, 1993; Poort et al., 2005) and higher-frequency ‘stadial to interstadial’ climatic oscillations (Kennett et al., 2003), in the debate of present-day climate change (e.g. Westbrook et al., 2009; Biastoch et al., 2011; Hunter et al., 2013; Marín-Moreno et al., 2013; Ruppel and Kessler, 2017), as well as in the context of the Paleocene-Eocene thermal maximum (PETM) and other past hyperthermal events (Dickens et al., 1995). Figure 1. The feedback interaction between climate change and gas hydrate dissociation: an initial change in climatic conditions (temperature, pressure or salinity) can cause hydrate dissociation. If the liberated gas (predominantly methane) eventually reaches the atmosphere, the increase in atmospheric greenhouse gas concentrations can lead to an additional warming, and maintain or reinforce further dissociation of hydrates. The present work reviews the first step in this cycle, which can be consideredACCEPTED as the sensitivity of gas hydrate reservoirs MANUSCRIPT to climate change. However, the role of gas hydrates in the carbon cycle and global climate, although being suggestive, remains controversial. A wide range of studies have contributed to the debate, presenting either field observations, measurements and/or quantitative modelling results in favor of or against a significant role for gas hydrates in controlling Earth’s climate. The discussion is complex because gas hydrate systems are governed by the interplay
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